READIT - 2009 - Indira Gandhi Centre for Atomic Research

Proceedings of READIT-2009, Kalpakkam - 603 102, 29-30 Dec 2009
web services reflect preconditions and effects through OWL ontologies. These comparative
strategies are discussed in the sixth section. The seventh section concludes with future
enhancements.
2. EXAMPLE SCENARIO
To understand service composition a web service scenario for booking in a travel
package from a travel agency is considered. We can see that a closed-world travel example
evolves into a dynamic integrated solution [4]. In a closed-world case the customer talk to the
travel agent who notes the customer request and generates a trip request document that may
contain several flight and hotel reservation. When done the agent puts the request in cancelled
or completed request database. The completed document is sent to the customer as response.
If cancelled the customer is intimated and the customer re-iterates. In the open-world variant
let us imagine that the travel agent wants to cooperate with external specialized service
providers that offer hotel and flight reservations. Upon receiving the customer’s request the
agent will create a trip request and derive the required hotel and flight reservation. Processes
can spawn off other processes at run-time.
3. DEFINITION
The process of creating composite services by composing web services is called
service composition. Composition involves invoking several services and combining their
functionality. The business logic of the client is implemented by composing several services.
Service composition is performed by composing elementary or composite services. Service
composition modeling can be done in six different dimensions. A component model is a basic
assumption. An orchestration model uses process-modeling languages like UML activity
diagram, Petri-nets, state-charts etc. Data and data access model define how data is specified
and exchanged between components. The service selection model deals with static and
dynamic binding. It deals with how a web service is selected as a component. A component
can be statically selected at design-time or dynamically selected at run-time. Transactions
model define which transactional semantics can be associated to the composition and how this
is done. The composite service must work without aborting during execution. The exceptional
states must be handled with an exceptional handling model.
4. COMPOSITIONAL ISSUES
Web service composition in itself poses as a complex and challenging. There are six
major issues that have a huge impact on service composition. These issues are discussed
below.
Coordination – When web services are composed to build composite applications
coordination of sequences of operations are needed to ensure correctness and consistency.
Transaction – Over the coordination framework a transaction protocol should be added to
ensure ACID properties of web service transactions in the long running business activities.
Context – Context is the information utilized by the web service to adjust execution and
output to provide the client with customized and personalized behavior. Context contains
information like customer’s name, address, current location, the type of client device and
preferences regarding communication.
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